In LTE (Long Term Evolution) as a telecommunications standard of the mobile phone, a system is configured to include RRE (Remote Radio Equipment) or the like, which shares an antenna with W-CDMA (Wideband Code Division Multiple Access) in order to improve workability at the time of installation and to reduce the running cost. When a base station wireless transmission device of one system based on an OFDM (Orthogonal Frequency Division Multiplexing) system such as LTE and the like, and a base station wireless transmission device of the other system are coupled to each other with use of a sharing antenna or the like, an interference wave from the wireless transmission system of the other system is input to the wireless transmission device of the one system, and serves as a disturbing wave. It is important to precisely measure VSWR (Voltage Standing Wave Ratio) even in the environment as described above, and to apply the measurement result on the system for controlling wireless communication.
FIG. 2 illustrates a VSWR measurement device and a VSWR measurement method in a first related art. A baseband signal generator 2 converts an input data signal generated in MAC 1 into an I/Q axis signal as a modulated baseband signal. Then, a modulator 3 modulates the I/Q axis signal into an RF-band modulated downlink signal, based on an oscillation signal from a local oscillator 40. A high output amplifier 4 amplifies the RF-band modulated downlink signal to a predetermined transmission power for output to a filter 5. The filter 5 reduces a transmitted spurious component of the RF-band modulated downlink signal. The RF-band modulated downlink signal is emitted to the space by an antenna 7. A VSWR measurement device precedes the antenna 7 in order to measure VSWR when the antenna 7 is installed, and to check that spatial emission from the antenna 7 is carried out without a problem. In this example, a directional coupler 6 is inserted to precede the antenna 7 in order to detect a traveling wave and a reflected wave for VSWR measurement.
The directional coupler 6 outputs a traveling wave to a traveling wave detector 8. Further, the directional coupler 6 outputs a reflected wave to a reflected wave detector 9. The traveling wave detector 8 detects a voltage value of a traveling wave. Further, the reflected wave detector 8 detects a voltage value of a reflected wave. A difference detector 10 detects a difference between the detected voltage value of the traveling wave and the detected voltage value of the reflected wave. A VSWR calculator 11 calculates VSWR, based on the difference between the detected voltage values, and a VSWR average unit 12 performs a smoothing process to the calculated VSWR. In the first related art, VSWR smoothed by the VSWR average unit 12 is output and displayed as a final report value. In the environment to be described later, however, a large error may be included in a measurement result of VSWR.
Next, a matter to be environmentally concerned is described. FIG. 5 illustrates a case, in which a base station wireless transmission device of one system based on an OFDM system such as LTE and the like, and a base station wireless transmission device of the other system are coupled to each other with use of a sharing antenna or the like. In this case, an interference wave from the wireless transmission device of the other system is input to the wireless transmission device of the one system.
FIG. 6 illustrates a time waveform of a downlink signal in LTE. Signals are densely present on the time axis in a test model such as E-TM1.1 or E-TM2. On the other hand, in a control channel signal whose density is the lowest among the actual operation signals (a state that no terminal link is present), broadcast information of a short duration, and a reference signal of a shorter duration, serving as a training signal on the terminal side, are sparsely present on the time axis within one frame (=10 subframes·10 msec).
In view of the above, a problem to be solved when VSWR measurement is performed in various environments by a configuration example of the VSWR measurement device illustrated in FIG. 2 is described. As illustrated in FIG. 7, when a test signal having a high density is used, the signal level fluctuates, but the measurement value of VSWR is always accurate (in this case, 1.5), because the fluctuation is within the dynamic range of a wave detector.
As illustrated in FIG. 8, when a downlink signal having a low density is used, the signal frequently repeats switching between an on-state and an off-state, and a large part of the signal may be lowered than the lower limit of the dynamic range of a wave detector. This may lower the precision of a measurement value of VSWR. On the other hand, during a time period when broadcast information is output, the signal density is high. Therefore, it is possible to obtain an accurate measurement value of VSWR.
Further, as illustrated in FIG. 9, when a backward disturbing wave from the other system is added to a downlink signal whose density is low during an operation, an error may be largely increased, because the disturbing wave may be misjudged as a reflected wave. Precision of a measurement value of VSWR may be considerably lowered depending on the level of the disturbing wave. However, during a time period when broadcast information is output, the measurement value of VSWR is accurate because the signal density is high.
As described above, in an actual environment, when a backward disturbing wave from the other system coupled with use of a sharing antenna is present in a state that the signal density is low, precision of a measurement value of VSWR may be considerably lowered. Further, a measurement result of VSWR when an antenna is installed may be misjudged to be total reflection. As a result, a monitor station may erroneously issue an alarm, and the one system may stop transmitting the wave.
The following are exemplified literatures disclosing the configuration of the VSWR measurement device in the field of first related art. These literatures, however, do not describe a circuitry configuration or consideration relating to a measurement error of VSWR, which may be generated by a backward interference wave/disturbing wave from the outside.                VSWR detection circuit and VSWR detection method (PTL 1, FIG. 1)        VSWR measurement circuit (PTL 2, FIG. 1)        VSWR monitor circuit (PTL 3, FIG. 1)        Antenna port monitor system and method thereof (PTL 4, FIG. 1)        
As an improvement of the first related art, the following literatures are exemplified which pay attention to a circuitry configuration or consideration relating to a measurement error of VSWR, which may be generated by a backward interference wave/disturbing wave from the outside.                Antenna monitor device (PTL 5, FIG. 1, FIG. 2)        Standing wave ratio measurement device (PTL 6)        Voltage standing wave ratio measurement device (PTL 7)        
The improvement associated with the aforementioned three literatures is disclosed in FIG. 1 of PTL 5. In this example, a directional coupler inserted in order to detect a traveling wave and a reflected wave for VSWR measurement is provided between a filter (band-pass filter) connected to an antenna, and a wireless transmission device. According to this configuration, out-of-band attenuation by the filter is expected. Further, disposing a band-pass filter between a traveling wave detector and a reflected wave detector which are branched from the directional coupler makes it possible to sufficiently suppress an out-of-band interference wave/disturbing wave before wave detection.
Another configuration example associated with the aforementioned three literatures is disclosed in FIG. 2 of PTL 5. In this example, a down converter is disposed on each of a traveling wave path and a reflected wave path which are branched from a directional coupler, and the frequencies of local oscillators of the down converters are differentiated from each other. According to this configuration, allowing a signal of the baseband frequency or of the IF frequency after conversion to pass through a narrower band filter having sharper characteristics, while changing the passing bandwidth for down conversion makes it possible to attenuate the out-of-band interference wave/disturbing wave steeper from the outside. Thus, an error in the measurement value of VSWR due to the interference wave/disturbing wave is reduced.
The advantages of the configurations described in the aforementioned three prior art literatures, however, are obtained when the frequency bandwidth of an interference wave/disturbing wave from the outside is away from an intended transmission bandwidth. On the other hand, when the transmission bandwidth of the local system and the interference bandwidth of another system are close to each other, or when the transmission bandwidth of the local system overlaps the interference bandwidth of another system, it may be difficult to achieve an intended frequency selectivity by the filter. Or, it is necessary to provide a filter having sharper characteristics in order to obtain the advantages. As a result, the degree of difficulty in designing may be considerably increased. This may limits a shape of the device, and may increase the cost. Further, when a filter of a fixed bandwidth is disposed in a wave detection system, it may be difficult to flexibly handle a frequency change.
Next, FIG. 3 and FIG. 4 illustrate a configuration example invented in association with the present application, although this example is not prior art with respect to the invention of the present application. As illustrated in FIG. 3, a baseband signal generator 2 converts an input data signal generated in MAC 1 into an I/Q axis signal as a modulated baseband signal. Then, a modulator 3 modulates the I/Q axis signal into an RF band modulated downlink signal, based on an oscillation signal from a local oscillator 40. A high output amplifier 4 amplifies the RF-band modulated downlink signal to a predetermined transmission power for output to a filter 5. The filter 5 reduces a transmitted spurious component of the RF-band modulated downlink signal. The RF-band modulated downlink signal is emitted to the space by an antenna 7. A VSWR measurement device precedes the antenna 7 in order to measure VSWR when the antenna 7 is installed, and to check that spatial emission from the antenna 7 is carried out without a problem. In this example, a directional coupler 6 inserted to precede the antenna 7 in order to detect a traveling wave and a reflected wave for VSWR measurement.
The directional coupler 6 outputs a traveling wave to a traveling wave detector 8. Further, the directional coupler 6 outputs a reflected wave to a reflected wave detector 9. The traveling wave detector 8 detects a voltage value of a traveling wave. Further, the reflected wave detector 8 detects a voltage value of a reflected wave. A difference detector 10 detects a difference between the detected voltage value of the traveling wave and the detected voltage value of the reflected wave. A VSWR calculator 11 calculates VSWR, based on the difference between the detected voltage values, and a VSWR average unit 12 performs a smoothing process to the calculated VSWR.
In the above example, it is necessary to determine whether the report value from the VSWR average unit 12 is probable. In this configuration example, by branching a baseband signal generated in the baseband signal generator 2 and inputting the branched signals to a baseband signal amplitude integrator 14, signal levels, so as to determine whether the signal density is high enough to measure VSWR with high precision, are accumulated. FIG. 4 illustrates a sequence of masking a report value of VSWR when the baseband level by a determination method based on a threshold value is low. In this sequence, 100 samples of measurement values of VSWR are acquired every 10 msec corresponding to one frame of a downlink signal in LTE, and the VSWR average unit 12 updates the average value of VSWR one time in a second. The baseband signal integrator 14 integrates the baseband levels for one second corresponding to 100 samples each requiring 10 msec without synchronization with the updating operation as described above. When it is determined that a time segment during which the accumulated value as an integration result is equal to or larger than a threshold value set by an amplitude integrated value determiner 15 is continued, the amplitude integrated value determiner 15 determines that the report value of latest VSWR (an output from the VSWR average unit 12) smoothed at the time of the determination is probable. In this case, a switch SW 13 is switched to the direction of outputting and displaying a measurement value of VSWR so as to report the measurement value of VSWR to the outside.
When it is determined that there is a time segment during which the accumulated value as an integration result by the baseband signal integrator 14 is equal to or smaller than the threshold value set by the amplitude integrated value determiner 15, the amplitude integrated value determiner 15 determines that the report value representing a measurement result of latest VSWR (an output from the VSWR average unit 12) to which a smoothing process is applied by the time when the determination is made is not probable. Then, the SW 13 is switched to the direction of displaying that a measurement result of VSWR is invalid so as to invalidate the report on the measurement result of VSWR to the outside.
In the above configuration example, however, it takes time from measuring the baseband level to determination. This may make it difficult to obtain a quick response. Further, when the transmission level is low, regardless that the density of a downlink signal is high, an integrated value does not exceed a threshold value and masking of the measurement result of VSWR may occur, regardless that VSWR can be measured with high precision, as well as a signal having a large instantaneous transmission power and low signal density.
PTL 8 discloses a method for checking not normality on a measurement result of VSWR when a transmission antenna is installed, but normality on a measurement value of VSWR when a receiving antenna is installed. Further, it is necessary to provide a PN (Pseudorandom Noise) spread signal generator and a demodulator individually and dedicatedly in a main signal receiving system in order to measure VSWR of a receiving antenna. This may limits a shape of the device, and may increase the cost. Further, a PN spread signal, which is different from an operation transmission signal, may be emitted from the receiving antenna as unwanted radiation when VSWR is measured. On the other hand, when a backward interference wave from the outside is present when a sharing antenna is used, in the course of deriving a value of VSWR as represented by VSWR=S1/(N+I′)−S2/(N+I), I or I′ in the denominator may be applied, so that increasing an error in VSWR is also may be a problem. Further, the denominator increases due to the large I or I′. This may make it difficult to secure a sufficient C/N (Carrier to Noise) ratio, and make it impossible to demodulate the PN.
In the technique described in PTL 9, in measurement of VSWR when an antenna (ANT) is connected, when the electrical length between the load of the antenna (ANT) and a coupling port (CPL) is different, and when a traveling wave component leaks to a reflected wave port due to poor directivity of CPL, an error may be included in a composite vector of a reflected wave, and an error may occur in a measurement result of VSWR. Further, disposing a phase unit between the antenna and the coupling port to change the electrical length makes it possible to remove a traveling wave leakage component from the measurement result of VSWR, based on computation of a vector of a maximum reflected wave and a vector of a minimum reflection for improvement of measurement precision of VSWR. Therefore, the example illustrated in PTL 9 provides improvement of measurement precision of VSWR in a state that an external interference wave is not present, and does not consider the density of a traveling wave or of an external interference wave.